The present invention relates to video signal playback systems and, more particularly, for apparatus whereby time-base errors that are introduced into a composite color video signal which is reproduced by a video signal playback system are compensated. A particular feature of this invention relates to equalizing the time-base errors in a chrominance component of the reproduced composite color video signal with the time-base errors which have been introduced into the luminance component.
In a typical video signal recording system, such as a video tape recorder (VTR), a composite color video signal is separated into its chrominance and luminance components, and these separated components generally are processed and recombined for recording in oblique tracks across a moving magnetic medium, such as magnetic tape. The recording transducer typically is formed of a pair of magnetic recording heads which are rotated, for example, at a speed of thirty rotations per second.
Once the magnetic medium, such as tape, has been recorded, the recorded composite color video signals are reproduced by a compatible video signal playback system. In many instances, the video signal playback system comprises the playback section of the video recorder. However, as frequently occurs, a different video playback system may be used to reproduce the previously recorded composite color video signals.
It is possible that, during playback, the rotary speed of the magnetic heads and/or the linear speed of the magnetic medium may differ from that during recording. Although servo systems generally are used to control the rotary speed of the magnetic heads and the linear speed of the medium, nevertheless, speed fluctuations may occur. Also, if magnetic tape is used as the recording medium, it is possible that this tape may be subjected to a change in dimension, such as tape shrinkage or stretching, subsequent to the recording operation. All of these possible variations result in errors in the frequency and phase of the recorded and reproduced composite color video signals. Such errors, known as time-base errors, cause undesirable observable effects in the ultimately reproduced color video picture. Such effects appear as jitter, brightness distortion and improper color display. If these effects are provided in the reproduced composite color video signal which is to be used in connection with the transmission of a television broadcast, it is important that the time-base errors be corrected.
In one type of video signal playback system, the previously recorded composite color video signal whose luminance and chrominance components had been separately processed, is reproduced and the reproduced luminance and the chrominance components are separated. The chrominance component, which had been frequency converted to a range of frequencies less than the original chrominance component frequency range is reconverted back to such original frequency range. To effect such frequency reconversion, a local controllable oscillator is locked to the frequency converted subcarrier included in the reproduced chrominance component. Thus, if the reproduced subcarrier contains any time-base errors, the frequency reconversion operation essentially eliminates such time-base errors. Then, the frequency reconverted chrominance component, whose time-base errors have been corrected, is combined with the luminance component so as to reform the original composite color video signal. However, although the time-base errors have been removed from the chrominance component, such time-base errors still may be present in the luminance component, including the vertical and horizontal synchronizing signals. Accordingly, in many instances, additional time-base correcting circuitry is relied upon to receive the composite color video signal which has been reproduced by the video signal playback system and to correct the time-base errors present in such composite color video signal. An example of one type of time-base correcting circuitry is disclosed in copending application Ser. No. 642,197, assigned to the assignee of the present invention.
Another example of time-base error correcting apparatus is described in U.S. Pat. No. 2,979,558. In that apparatus, the burst signal included in a reproduced composite color video signal is separated and used to synchronize, or lock, an oscillator, such as the so-called start-stop oscillator. Hence, in the event that the time-base errors in the chrominance component reproduced by the video playback system have not been corrected, the output from this oscillator will contain such errors. The output from this oscillator is used to produce a frequency converting signal which also contains the time-base errors. This frequency converting signal then converts the frequency of the chrominance component, whereby the time-base errors in the frequency converting signal and the time-base errors in the chrominance component are substantially equal and cancel each other. However, in this prior art system, there is no recognition of the time-base errors which also are present in the luminance component, and particularly in the horizontal synchronizing signal which is included in such luminance component. Furthermore, by locking the local oscillator to the separated burst signal, the time-base errors which are imparted into the oscillator output lag the actual time-base errors which are contained in the chrominance component by one horizontal line interval. This is because the burst signal is present only at the beginning of the horizontal line interval and, therefore, the local oscillator is provided with the time-base errors which are present only during the beginning portion of that horizontal line interval. While these time-base errors may change over the duration of the horizontal line interval, the locked oscillator cannot follow such changes.
In another proposal for correcting time-base errors which may be present in the chrominance component of a reproduced color video signal, as described in U.S. Pat. No. 3,075,041, the color information which is modulated onto a subcarrier is demodulated, or decoded, therefrom and then is remodulated, or encoded, onto another subcarrier. If the phase or frequency of the reproduced subcarrier changes because of time-base errors, it is necessary to generate a local reference signal which is synchronized with the reproduced subcarrier and to use this local reference signal for decoding the color information. Once decoded, a local subcarrier generated by a highly regulated oscillator, such as a crystal oscillator, is used for encoding. However, this system suffers from the same disadvantage found in the aforenoted system, viz., the local reference signal is generated by a so-called color lock oscillator which is controlled by the burst signal included in the reproduced chrominance component, and therefore lags the time-base errors in the chrominance subcarrier by one horizontal line interval.